Conventional pumping stages known in the vacuum field are obtained through the co-operation between a rotating cylindrical or frusto-conical rotor surface and a stationary, similarly cylindrical or frusto-conical stator surface, one or more axially extending grooves, e.g. helical grooves, being formed on one of the facing surfaces of the rotor or the stator.
Pumping stages of this structure is known in the art as Holweck stages. Pumping stages are widely employed in particular for low vacuum ranges in the so-called “backing pumps”, downstream high-vacuum pumping stages, for instance turbo-molecular pumping stages. An example of such a vacuum pump is disclosed in the U.S. Pat. No. 1,492,486.
By alternating a plurality of coaxial cylindrical or frusto-conical rotors and stators, and by forming one ore more helical grooves on each wall of a pair of facing walls of the rotors or the stators, a plurality of Holweck pumping stages arranged in series or in parallel can be obtained, as shown, for example, in patents EP 260,733 and U.S. Pat. No. 2,730,297.
According to the prior art, the helical grooves are generally obtained by means of a mechanical working of a metal cylinder, for instance by milling.
The grooves may have variously shaped (e.g. rectangular, trapezoidal, etc.) profiles, may have variable sizes or, may have variable helix pitches. In view of different requirements for shape and sizes, forming these grooves by milling is very difficult and expensive.
It is the main object of the present invention to overcome the above drawback. A process is provided that allows for obtaining stators for pumping stages with substantially axially extending grooves, in quick and inexpensive manner.
It is another object of the invention to provide a process for easily manufacturing stators for pumping stages with axially extending grooves, for instance helical grooves of Holweck type with different characteristics.
It is a further object of the invention to provide with improved assembly and maintenance procedures.